1. Field of the Invention
The present invention relates to an alternator driven by an internal combustion engine, for example, and in particular, relates to a stator construction for an automotive alternator mounted to an automotive vehicle such as a passenger car or a truck.
2. Description of the Related Art
In recent years, compact high output and improvements in quality have been increasingly required of alternators. In order to achieve compact high output, it is important to design constructions which distribute magnetic loading and electrical loading in the most appropriate manner, and with the highest possible density within a limited volume.
For example, as engine compartments become progressively smaller, mounting space for automotive alternators is becoming less and less free, but at the same time, improvements in alternator output are required because of increases in automotive vehicle loads. Furthermore, there is increased need for noise reduction both inside and outside automotive vehicles, and although engine noise has been reduced, noise from automotive alternators, which run constantly to generate electricity in order to supply the electrical load for the vehicle, has remained a problem. Fan noise and magnetic noise in particular have been problems in automotive alternators which operate in a wide range of rotational frequencies from low to high speed.
Because automotive alternators are constantly generating electricity, they generate a lot of heat due to the joule heat of the output electric current and are subjected to a severe heat environment, requiring extremely high heat resistance.
Furthermore, within the engine compartment, an automotive alternator is often mounted directly onto an engine, where it is exposed to liquids such as engine oil and antifreeze, in addition to rain water, salt water, mud, etc., conditions where the corrosive environment is extremely severe. There are problems due to corrosion which lead to interruptions in power generation, etc., but most causes of interruption to power generation are the result of damage to insulation on a winding which occurs during the process of manufacturing a stator, or due to electrical short-circuiting in structurally exposed portions of the winding.
For compact high output by alternators, in particular, it is necessary to increase the space factor of electrical conductors housed within magnetic circuits of the stator, and to line up and increase the density of bridge portions of the stator winding (bridge portions outside a stator core are called coil ends), and in addition to this, various improvements have been proposed in order to answer the requirements for low noise, heat-resistance, corrosion resistance, etc., mentioned above. Constructions aimed at increasing the space factor of the electrical conductors using short conductor segments for the electrical conductors in the stator or aimed at lining up and increasing the density of the coil ends have been proposed in the publication of WO 92/06527 and in Japanese Patent No. 2927288, for example.
FIG. 29 is a side elevation showing part of a stator of a conventional automotive alternator such as described in Japanese Patent No. 2927288, for example, FIG. 30 is a perspective showing a conductor segment used in the stator of the conventional automotive alternator shown in FIG. 29, and FIGS. 31 and 32 are perspectives from a front end and a rear end, respectively, of part of the stator of the conventional automotive alternator shown in FIG. 29.
In FIGS. 29 to 32, the stator 50 includes: a stator core 51; a stator winding 52 wound onto the stator core 51; and insulators 53 mounted inside slots 51a, the insulators 53 insulating the stator winding 52 from the stator core 51. The stator core 51 is a cylindrical laminated core laminated by stacking thin steel plates, and has a number of slots 51a extending axially disposed at even pitch circumferentially so as to be open on an inner circumferential side. In this case, ninety-six slots 51a are formed so as to house two sets of three-phase winding portions such that the number of slots housing each phase of the winding portions corresponds to the number of magnetic poles (sixteen) in a rotor (not shown). The stator winding 52 is constructed by joining a number of short conductor segments 54 in a predetermined winding pattern.
The conductor segments 54 are formed into a general U shape from an insulated copper wire material having a rectangular cross section, and are inserted two at a time from an axial rear end into pairs of slots 51a six slots apart (a pitch of one magnetic pole). Then, end portions of the conductor segments 54 extending outwards at a front end are joined to each other to constitute the stator winding 52.
More specifically, in pairs of slots 15a six slots apart, first conductor segments 54 are inserted from the rear end into first positions from an outer circumferential side within first slots 51a and into second positions from the outer circumferential side within second slots 51a, and second conductor segments 54 are inserted from the rear end into third positions from the outer circumferential side within the first slots 51a and into fourth positions from the outer circumferential side within the second slots 51a. Thus, within each slot 15a, four straight portions 54a of the conductor segments 54 are arranged to line up in a row in a radial direction.
Then, end portions 54b of the conductor segments 54 extending outwards at the front end from the first positions from the outer circumferential side within the first slots 51a and end portions 54b of the conductor segments 54 extending outwards at the front end from the second positions from the outer circumferential side within the second slots 51a six slots away in a clockwise direction from the first slots 51a are joined to form an outer layer winding having two turns. In addition, end portions 54b of the conductor segments 54 extending outwards at the front end from the third positions from the outer circumferential side within the first slots 51a and end portions 54b of the conductor segments 54 extending outwards at the front end from the fourth positions from the outer circumferential side within the second slots 51a six slots away in a clockwise direction from the first slots 51a are joined to form an inner layer winding having two turns.
In addition, the inner layer winding and outer layer winding constituted by the conductor segments 54 inserted into the pairs of slots 51a six slots apart are connected in series to form one phase of the stator winding 52 having four turns.
A total of six phases of the stator winding 52 each having four turns are formed in this manner. Then, two sets of three-phase stator winding portions are constructed by connecting three phases each of the stator winding 52 into alternating current connections.
In the conventional stator 50 constructed in this manner, at the rear end of the stator core 51, turn portions 54c of the pairs of conductor segments 54 inserted into the same pairs of slots 15a are lined up in rows in a radial direction. As a result, the turn portions 54c are arranged in two rows circumferentially to constitute a rear-end coil end group.
At the front end of the stator core 51, on the other hand, joint portions formed by joining the end portions 54b of the conductor segments 54 extending outwards at the front end from the first positions from the outer circumferential side within the first slots 51a and the end portions 54b of the conductor segments 54 extending outwards at the front end from the second positions from the outer circumferential side within the second slots 51a six slots away, and joint portions formed by joining the end portions 54b of the conductor segments 54 extending outwards at the front end from the third positions from the outer circumferential side within the first slots 51a and the end portions 54b of the conductor segments 54 extending outwards at the front end from the fourth positions from the outer circumferential side within the second slots 51a six slots away are arranged to line up radially. As a result, joint portions formed by joining end portions 54b to each other are arranged in two rows circumferentially to constitute a front-end coil end group.
In the stator 50 of the conventional automotive alternator, as explained above, the stator winding 52 is constructed by inserting short conductor segments 54 formed in the general U shape into the slots 51a of the stator core 51 from the rear end, and joining end portions 54b of the conductor segments 54 extending outwards at the front end.
Thus, because the front-end coil end group is constructed by circumferentially arranging the joint portions formed by joining the end portions 54b, which have lost their insulation due to soldering or welding, the coil-end construction is easily corroded by exposure to moisture, making corrosion resistance extremely low.
Furthermore, because the front-end coil end group is composed of two rows of ninety-six joint portions, i.e., 192 joint portions, the construction facilitates short-circuiting between the joint portions, increasing the likelihood of short-circuiting accidents.
A large number of the short conductor segments 54 must be inserted into the stator core 51 and their end portions 54b must be joined by welding, soldering, etc., significantly reducing operability. Furthermore, the amount of each conductor segment 54 which is inserted into the slots 51a must be greater than the length of the stator core 51, facilitating damage to the insulation and reducing the quality of the finished product. In addition, when joining the end portions 54b, short-circuiting often occurs between the joint portions due to spilt solder or weld melt, significantly decreasing mass-producibility.
The end portions 54b of the conductor segments 54 are joined to each other by clamping a portion thereof in a jig, and soldering or welding the tips thereof. Thus, because clamping area is required for the jig and expansion of the soldered portions or welded portions occurs, the height of the coil ends is increased and space between the joint portions is reduced. As a result, coil leakage reactance in the coil end portions is increased, causing output to deteriorate, and wind resistance is increased, exacerbating wind noise.
Furthermore, as a measure against magnetic noise, mutual cancellation of magnetic pulsation forces by winding two sets of three-phase windings into slots in positions offset by an electrical phase difference of 30xc2x0 has been proposed in Japanese Patent Laid-Open No. HEI 4-26345, for example. However, when attempts are made to adopt this example of improvement of magnetic noise in small stators, the slot spacing becomes extremely narrow because twice as many slots are required. Thus, it was not possible to apply general winding methods in which a stator winding is constructed by preparing an annular coil by winding continuous wire into an annular shape, then, preparing a star coil by deforming this annular coil into a star shape, then installing straight portions of the star coil into the slots of the stator core. Furthermore, the above-mentioned winding method using the conductor segments 54 could not be applied because buckling, etc., of the conductor segments 54 occurs during insertion into the slots. Additionally, when welding the end portions 54b of the conductor segments 54 to each other, the conductor segments 54 are softened by temperature increases during welding, reducing the rigidity of the stator and decreasing the effective reduction in magnetic noise.
In addition, it is necessary to answer the various demands of automotive alternator output with electromagnetic design. In particular, in order to improve output of an alternator in a low-speed frequency region in response to idling frequency in an automotive engine, it is necessary to shift an output commencement frequency to the low-speed side. To this end, it is necessary to improve the voltage induced by the alternator by increasing magnetomotive force, that is, electric current supplied to a field coil, or by increasing the total conductor count, that is, the number of turns in the stator. Now, with the former, the output commencement frequency can be shifted to the low-speed side by increasing the supply of electric current to the field coil, but this is limited by reductions in the saturation of the magnetic circuits. With the latter, the output commencement frequency can be shifted to the low-speed side by increasing the number of turns, but when attempts are made to increase the total conductor count in a winding based on conductor segments 54, the number of joint portions increases proportionately, leaving no space for joining, and excessive increases in the number of turns cannot be practically applied.
The present invention aims to solve the above problems with the conventional art and an object of the present invention is to provide an alternator having both high serviceability and productivity, capable of satisfying the performance and quality commonly required of today""s alternators.
An additional object of the present invention is to provide an alternator applicable to automotive use which has compactness, high output, and low noise.
In order to achieve the above object, according to one aspect of the present invention, there is provided an alternator comprising:
a rotor for forming north-seeking (N) and south-seeking (S) poles along a rotational circumference;
a stator comprising:
a stator core disposed facing the rotor; and
a polyphase stator winding installed in the stator core; and
a bracket supporting the rotor and the stator,
wherein the stator core comprises a laminated iron core formed with a number of slots extending axially at a predetermined pitch in a circumferential direction,
the polyphase stator winding comprises a number of winding sub-portions in each which a long strand of wire is wound so as to alternately occupy an inner layer and an outer layer in a slot depth direction within the slots at intervals of a predetermined number of slots, the strand of wire folding back outside the slots at axial end surfaces of the stator core to form turn portions,
the turn portions align in a circumferential direction to constitute coil end groups at both axial end portions of the stator core, and
the turn portions constituting the coil end groups at both axial end portions of the stator core are provided with an insulative coating.